Open the Settings app
Tap Sounds & Haptics
Turn on Silent Mode using the Ring/Silent switch on the side of the iPhone
Use Do Not Disturb or Focus mode to silence incoming calls
Go to Settings
Tap Focus
Tap Do Not Disturb
Turn on Do Not Disturb
Allow calls only from selected contacts if needed
Use the volume buttons to lower the ringer volume
Set a custom ringtone to silent if desired
Enable Silence Unknown Callers in Settings
Go to Phone
Tap Silence Unknown Callers
Turn it on
Use Airplane Mode to stop all calls and notifications
Turn on vibrate only or mute in the Control Center if available